This paper investigates the efficacy of a punishment mechanism in promoting cooperative behaviour in a public goods game when enforcement of punishment is uncertain. Experimental studies have found that a sanctioning system can induce individuals to adopt behaviour deemed as socially acceptable. Yet, our experiment shows that a sanctioning system cannot promote cooperative behaviour if enforcement is a low-probability event and free-riding behaviour is not often punished. This supports the view that punishment needs to be exercised to be feared, otherwise the simple threat of it cannot be effective in promoting cooperation.